Hybrid transactional memory (hybrid tm)

ABSTRACT

Embodiments related to a hardware transactional memory (HTM). An aspect includes setting a mode register of a processor core of a computer to indicate a HTM mode. Another aspect includes executing a plurality of transactions by the processor core in the HTM mode based on the mode register. Another aspect includes determining whether a first transaction of the plurality of transactions exceeds a failure limit of the processor core in the HTM mode. Yet another aspect includes, based on determining that the first transaction exceeds the failure limit of the processor core in the HTM mode, transitioning the processor to an assisted transaction mode by setting the mode register of the processor core to indicate the assisted transaction mode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 12/619,093, filed on Nov. 16, 2009, which is herein incorporated by reference in its entirety.

BACKGROUND

This disclosure relates to a computer processing system for transactional memory operations, and in particular to a Hybrid Transactional Memory System (Hybrid™).

Multicore microprocessors, as Intel's and AMD's dual core and quad core chips or Sun Microsystems's Niagara or RockH™, known as HTMs using large numbers of hardware threads of execution are currently being used in computing servers. The server software industry is moving towards new programming techniques to increase the concurrency of existing software in order to allow applications to take advantage of the large number of threads.

A Hybrid Transactional Memory System (Hybrid™) has been thought to provide a good tradeoff between hardware complexity and cost vs. overall system performance when executing transactions.

Most existing Hybrid™ systems provide hardware support as a Hardware Transactional Memory (HTM) like the illustrated CPU microprocessor core element in FIG. 1 of this application for some transactions and use a Software Transactional Memory (STM) system to handle others. To be cost effective, the hardware is limited in the number of storage locations that can be tracked. An example of a Hybrid™ system is one which can execute short transactions very quickly using a small amount of hardware but uses an STM system for long transactions that contain many storage references. The hardware may also be limited in the types of system operations that can be used while executing a transaction. These limitations cause some transaction to fail repeatedly. Hybrid™ systems must deal with these failures in some manner.

A significant drawback of these Hybrid™ systems is the fact that only code that has been generated by a Transactional Memory capable compiler can be used The code inside of transactions needs to be instrumented with calls to the STM software interface and therefore must be compiled together with STM in order to know which loads and stores should be instrumented. Dynamically compiled code can get around this limitation but carries other limitations related to speed and efficiency of compilation. Another drawback is that existing STM implementations have very long pathlengths and are therefore slow. This becomes a performance problem for applications which use Transactional Memory and fall back to the STM support often. Nevertheless, most recently Hybrid™ systems have been developed which exploit STM on HTM systems and provide for executing, one or more transactions in a first transactional memory mode while all transactions executed in the execution environment are executed according to a particular transactional memory implementation; and they try to dynamically transition to a second transactional memory mode during runtime; executing one or more transactions in the second transactional memory mode, while in said second transactional memory mode all transactions are executed according to a different transactional memory implementation than in the first transactional memory mode; where either the particular transactional memory implementation or the different transactional memory implementation is a hybrid transactional memory implementation in which some conflicts in the shared transactional memory space between transactions are resolved by a full STM software implementation without hardware support and other conflicts in the shared transactional memory space between transactions are resolved by a HTM hardware implementation.

Another alternative type of Hybrid™ system is one that doesn't require a full STM system. In this alternative the software programmer is required to provide both a Transactional Memory software path and an additional lock based path for their code. If a transaction fails repeatedly, the system forces the use of the lock based path. Although this type of system does allow the use of a limited hardware transactional memory (HTM) design, it is not desirable to build a lock based path for all the transactional code.

A third method of providing a backup for a hardware transactional memory HTM system that provides limited hardware resources. It has been suggested that the HTM system can “stop the world” and let one problem transaction run by itself. This has been referred to as granting one “favored” transaction. This solution will not work well for systems that provide dozens of threads of execution.

SUMMARY

The current invention uses a combination of hardware and software to provide improved support for a transactional memory system that has a simple/cost effective hardware design that can deal with limited hardware resources, yet one which has a back up that can still allow transactions to reference existing libraries, and which will not make a user code up a second lock based solution, and not force “stop the world” operations.

In accordance with the new transactional memory system design, an “assist thread” is used to improve a Hybrid Transactional Memory System (Hybrid™). The system uses a combination of an “assist thread”, additional hardware in the microprocessor, and a new software lock table, a Transaction Conflict Detection Table (TCDT), as an alternative replacement for the prior art a Software Transactional Memory (STM) system. The combination of the assist thread and software lock table allow the invention to achieve the same benefits of the prior-art use of a fallback STM without the high software overhead associated with STMs.

The new Transaction Conflict Detection Table (TCDT) managed by the assist thread facility is provided for the operating system (OS or Hypervisor). The table is used to provide locks on particular memory addresses currently in use by all transactions in the system. The current invention uses this new software table when the Hardware Transactional Memory (HTM) hardware is unable to process transactions due to any number of reasons (such as a cache overflow or unsupported operation).

One assist thread is provided for each CPU microprocessor core in the system. The assist thread is provided access to all of the addresses being processed in the load/store unit of the core. This flow of address information allows the assist thread to manage the new software lock table and provide “fallback” locks for all transactions in the system.

In the current invention, the assist thread is only used when the HTM system is unable to handle a particular transaction. When a single HTM based transaction exceeds the limits of the hardware, the whole system must transition to the use of the assist threads. The current invention provides a method to achieve this Hybrid™ behavior.

System and computer program products corresponding to the above-summarized methods are also described and claimed herein.

Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings.

As a result of the summarized invention, technically we have achieved a solution which provides a method for fault injection verification as a computer process without manual intervention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates one example a Hybrid Transactional Memory System.

FIG. 2 illustrates one example of nested transactional code.

FIG. 3 shows the hybrid transactional memory assist thread facility elements of the Hybrid Transactional Memory System added to the multi-chip module CPU supporting the new Hybrid™.

FIG. 4 shows the assist thread facilty's control of the setting of the TM Mode for a CPU.

FIG. 5 shows the format of the Transaction Conflict Detection Table (TCDT).

FIG. 6 illustrates how the assist thread and the TCDT are used together to provide a conflict detection function.

FIG. 7 shows assist thread execution of a transaction.

The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings in greater detail, it will be appreciated that in FIG. 1 there is illustrated the computing system illustrating an embodiment of a Hybrid Transactional Memory System which is illustrated by use of a plurality of CPUs as a multiprocessor system with modules 1 . . . n, illustrated by CPUs 1 . . n (100, 101 . . . 102), coupled to a system physical memory array (120) as well as other I/O devices (122) via a high speed interconnection element (110). Like other CPUs today, a CPU(computer processing unit) is integrated as in a multi-chip module with microprocessors which contain or are coupled in the CPU module to a memory controller, cache controllers, cache memory, and other components which form part of the CPU which connects to the high speed interconnect. The high speed interconnect functions under the architecture and operating system to interconnect elements of the computer system with physical memory, various I/O, devices and the other CPUs of the system. An example of such a system is U.S. patent Ser. No. 11/928758 filed Oct 30, 2007 entitled “Transactional Memory System which employs thread assists using address history tables”, which is a continuation-in-part of application 11/748,044 filed on May 14, 2007, both applications being fully incorporated herein by reference in their entirety. The current invention however does not require the PTRAN features of those earlier applications (although they can be present) but provides improvements for the CPU modules which are described in FIGS. 3-7 herein. Here a hybrid transactional memory system, comprising a plurality of the CPU microprocessors uses hardware transactional memory (HTM) transactions having transactional memory space accessible by the one or more of said CPU microprocessors via a high speed interconnect uses the hybrid transactional memory system's (Hybrid™) assist thread facility and software lock table coordination mechanism of FIGS. 3, 4, and 5 configured to coordinate execution of transactions over the shared transactional memory space without the use of a full software transactional memory (STM).

In the hardware assist thread facility of the Hybrid Transactional Memory System illustrated by FIG. 3, the CPU multi-chip modules includes either as part of the core microprocessors contained therein or as an assist chip the additional elements illustrated by the CPU 100, additional assist thread facility hardware for each microprocessor core as detailed in FIG. 3, which is also duplicated in other CPUs 1 . . . n (101 and 102), so they have new counters and registers enabling the features described herein.

As illustrated in FIG. 1 the physical memory array stores transaction data (121) in real system memory. The high speed interconnection element (110) can be implemented as a shared bus using IBM's elastic interface or by crossbar switch. The invention applies to systems which use any other scheme of interconnection physical memory to a multiprocessor system which may be implemented in one or more chips which provide the core microprocessors. The memory could be broken down into smaller portions and distributed across private connections to each of the CPU chips as done for the IBM Systems using the Power4 microprocessor or for some Intel and AMD Opteron based servers. The CPU microprocessors and memory controllers may be located together on a single silicon chip or they may be spread across multiple chips, but in the preferred embodiment each multi-chip module for a CPU has its own memory controller. The physical memory of the computing system is divided into n increments

The Hybrid Transactional Memory System allows programmers to use their own transactional code and to include calls to existing software libraries inside of their transactions, and to use instructions used in other transactional memory systems. For instance, the nested routine illustrated in FIG. 2 can be executed. Instructions (TBEGIN, TEND) as illustrated by the Nested code sequence of FIG. 2 are used to mark the beginning and end of a group of instructions. The instructions which execute between the new instructions are sometimes referred to as an “atomic instruction group” (AIG) illustrated by threads of instructions shown in FIG. 2 (Instruction A0, Instruction A1, Instruction A2). The instruction sequence also can be nested threads, as specifically shown in FIG. 2, where a call can be included.

Additional storage access rules are used when a processor is executing instructions which are part of an atomic instruction group or threads. The system provides a way for hardware to quickly detect potential conflicts between the storage accesses of multiple instruction groups or threads. Although the system uses special new instructions such as BEGIN and END (TBEGIN and TEND) to mark the boundaries of a transaction, any other method could be used to identify a group of memory locations that are to be updated in an atomic fashion. The system is compatible with any number of software interfaces that may be used to implement a transactional memory system. The system can provide the same fast conflict detection for any system which is attempting to provide an atomic update of multiple storage locations. This also applies to systems which mark the boundaries of an instruction group in any other ways, including compiler generated hints attached to other instructions, internal microprocessor commands generated by internal microcode or millicode.

FIGS. 3 and 4 illustrate the hardware and function of the assist thread facility, which manages the software table of FIG. 5. One assist thread facility is provided for each CPU microprocessor core in the system. The assist thread is provided access to all of the addresses being processed in the load/store unit of the core. This flow of address information allows the assist thread facility to manage the new software lock table and provide “fallback” locks for all transactions in the system.

In the current invention, an assist thread is only used when the HTM system is unable to handle a particular transaction. When a single HTM based transaction exceeds the limits of the hardware, the whole system must transition to the use of the assist threads. The current invention provides a method to achieve this Hybrid™ behavior.

Turning now to the drawings in greater detail, it will be seen that in FIG. 3 illustrates one of the microprocessor CPUs of a multiprocessor system with a plurality of CPUs 1 . . . n, each connected to a bus, as in FIG. 1, in accordance with this improved computer system using a high speed interconnect.

As illustrated in FIG. 3, which shows the elements of a CPU 100, provides for each of the microprocessor cores of the CPU 100 an additional assist thread facility elements, or an additional assist thread facility chip for the CPU, as the illustrated part of the CPU additional hybrid transactional memory assist thread facility elements illustrated below the traditional CPU elements (100) (microprocessor cores, cache, cache controller, clock controller, memory controller, load/store control logic, interface) provided in FIG. 3. The assist thread facility of each CPU includes a HTM cache (301) memory for the CPU (such as 100 which has included therein a memory controller, cache controllers, cache memory, and other components which form part of the CPU which connects to the high speed interconnect 110), the transactional facility control logic (302) and an Address History Table (303) coupled to load and store units for writing information entries of the microprocessor (100) in the AHT by the CPU's memory controller load store controls and coupled via instruction execution pipelines of the CPU (100) for writing information into the AHT. The Transaction Facility Control Logic (302) enables the transactional memory hardware to move into an assisted transaction mode of operation using the hardware and software controls of the assist thread's transaction facility control logic (302).

Each CPU has the ability to execute transactional memory instructions TBEGIN and TEND. The CPU's HTM memory cache (301) has a hardware cache design that supports under the logic provided by the Transactional Facility Control Logic (302) the ability to detect conflicts, abort failed transactions and quickly discard partially completed transactional results.

A TM mode register (304) of the hybrid transactional memory elements of the assist thread facility for a CPU 100 is added which enables the CPU to treat transactional memory operations in 2 different ways. When the mode register is set to 0, the CPU uses a Hardware Transactional Memory (HTM) method restricting execution only to hardware. When the mode register is set to 1, the CPU operates in an “Assisted Transaction Mode”. The control of the setting of the TM Mode is described in detail in FIG. 4.

The hybrid transactional memory elements of a CPU also contains a register (305) bi-directionally coupled to the Transactional Facility Control Logic (302) to be used as a Transaction Failure Counter (305). In the preferred embodiment a special purpose register (305) is provided for this purpose, other implementations of the hybrid transactional memory elements may use a location in main memory to hold the count or may provide another form of scratch registers.

The hybrid transactional memory elements for a CPU 100 also includes a Transactional Failure Limit (306) which is another special purpose register (306) coupled to the Transactional Facility Control Logic (302) to be used as a Transaction Failure Limit (306). Again, other implementations of the hybrid transactional memory elements may use a location in main memory to hold the limit or may provide another form of scratch registers.

Turning now to FIG. 4, it will be seen that when a transaction is executed for the first time, the Transaction Failure Count is set to 0 (200). The execution of the transaction is started (201) in hardware only mode (HTM). A decision (202) is made based on the outcome of the transaction execution. If the hardware does not detect a conflict for the storage locations accessed by the transaction and the transaction does not exceed the hardware limits and the transaction does not attempt to use system operations that are prohibited inside of transactions, the transactions is declared “successful” and the results of the instructions executing in the transaction are “committed” (203) to the architectural state of the system. If a conflict was detected or an abort was indicated, the transaction fails and the Transaction Failure Count register is incremented (204) and the temporary results of the partially executed transaction are discarded. The Failure Count is then compared against a Failure Count Limit recorded in a register (306) for that purpose and a decision (205) is made based upon this comparison. If the Failure Count does not exceed the limit the transaction is tried again (201) in the same Hardware HTM Mode.

If the Failure Count exceeds the limit value in the Transactional Facility Control Logic (302) stored in the Transaction Failure Limit (306) then the TM Mode is changed to 1 and the CPU enters the Assisted Transaction Mode (207). In the preferred embodiment this change of mode includes the starting of the “assist thread of execution” (208). This is accomplished via an operating system interface. Other embodiments may leave the assist thread active in the pool of threads.

While operating in Assisted Transaction Mode, a software table is used to keep track of possible conflicts between transactions. In the preferred embodiment one table is used for all transactions. As has been noted before, the new Transaction Conflict Detection Table (TCDT) managed by the assist thread facility is provided for the operating system (OS or Hypervisor). The table is used to provide locks on particular memory addresses currently in use by all transactions in the system. The current invention uses this new software table when the Hardware Transactional Memory (HTM) hardware is unable to process transactions due to any number of reasons (such as a cache overflow or unsupported operation).

The table may be cached, hashed, duplicated, and shadowed. FIG. 5 shows the format of this Transaction Conflict Detection Table (TCDT). One row of the TCDT is used for each real page (4 KB) of system memory. Other embodiments may use a different granularity. The optimal choice of TCDT row granularity depends on many factors including expected transaction behavior and system memory size.

In the preferred embodiment, each row of the TCDT has room for four Process IDs (only 3 are shown) (320 a,320 b,320 c) included with the Process ID is an indication of whether that process made the entry for a “favored” transaction. A read/write indication and a line number is provided for each Process ID (330 a,330 b,330 c). The line number to indicate which line in the page has been referenced by the associated process who's id is written into the Process ID field. (A line number of xxxx indicates that the process has registered an interest in the whole page). These entries provide the ability for a transaction to “register” its read/write set into the TCDT. The granularity of the table does not need to match the granularity of the hardware TM detection.

Now, the current invention uses this assist thread facility and the TCDT table when the Hardware Transactional Memory (HTM) hardware is unable to process transactions due to any number of reasons, like cache overflow and unsupported operations.

FIG. 6 shows how the assist thread facility and the TCDT are used together to provide a conflict detection function. This software table based conflict detection is used as a backup to the limited hardware based transactional memory conflict detection. The steps of FIG. 6 are only used for transactions that have failed repeatedly in the hardware TM mode. The assist thread of the assist thread facility operates as described in the referenced U.S. patent Ser. No. 11/928758 filed Oct 30, 2007 entitled “Transactional Memory System which employs thread assists using address history tables”. Thus, after a TBEGIN instructions is executed, the addresses are written into the Address History Table, and also, the addresses are pulled out of the AHT by the assist thread and processed.

Each address is examined (400) and it is determined whether the system has reached the end of the current transaction (401). If the end of the transaction has been reached the temporary results of the transaction are committed (406). The preferred embodiment places the new data in main storage so that the commit is very fast.

But, in the current embodiment, if the end of a transaction has not been reached then the Transaction Conflict Detection Table (TCDT) is accessed (402) using the high order address bits of the address that was read out of the AHT. It is now determined whether a new entry is required in the TCDT for this address. The high order bits of the address are used to index into the table and a read of a row. The current embodiment uses one row per 4K main storage page. If the row is empty (403) then the process ID of the owning transaction is placed in the table along with an indication of whether the process is “favored”. If the row already has an entry, the entry is examined further. Is it then determined (404) whether the current transaction is “favored”. If the current transaction is favored then the TCDT entry for it is marked as favored (405). Any other processes that have a write entry for the same line in the page are killed. If the current update is for a write then it is necessary to kill all transactions that have registered either a read or a write for the same line.

If the current transaction is not favored then the assist thread then determines whether the address is from a write (408). If it is a write, the TCDT entry is examined to determine if there is already an entry from another process ID for the same line. If there is then the current transaction fails. If there is not then a new entry is made for the current process. If there isn't room, the transaction fails.

If the address is not from a write it is determined whether there is room in the TCDT (410). If there is room, the current process ID is added to the TCDT entry (411). The assist thread then continues with the next AHT entry (400).

When the transaction is completed the entries in the table are removed. A per-transaction list of all the addresses touched is used to quickly find the entries that need to be removed. They can be removed in a “lazy” fashion, the only impact will be an over-indication of the conflict cases and some additional use of the favored transaction mechanism.

FIG. 6 showed that there are several ways for the assisted transaction execution to fail, mainly due to conflicts over the read and write set of multiple transactions executing concurrently in the system. Repeated failures may cause a deadlock condition. FIG. 7 shows how the current invention prevents deadlocks by using the favored transaction state and the favored entries in the TCDT.

Turning now to FIG. 7, it will be seen that the assist thread execution of a transaction begins with the resetting of the Transaction Failure Count to 0 (500). The Transaction Failure Count may be kept in a register or in memory that is accessed by the assist thread. Execution of a transaction only gets to this point after the HTM system has failed repeatedly. The system then executes the transaction in the Assist Thread Mode (501) using the steps previously described in FIG. 6. It is determined whether the assisted execution of the transaction is successful (502), if so the temporary state of the transaction is committed to the architectural state of the system (503). If the assisted execution fails then the Failure Count is incremented (504). A decision is made (505) based on the Failure Count and a Limit. If the limit has not been reached the transaction is executed again (501). If the limits is reached after repeated failures of the assisted execution of the transaction the assist thread will request a “Favored Transaction Token” (FTT) (506). Only one process thread in the system is granted the FTT at one time. If the token is not currently available (507) and is therefore not received the assist thread will wait until the token is received (508). Once the token is received, execution continues (509) and the current transaction is guaranteed to succeed. A process which holds the FTT while performing an assisted transaction execution is favored during all conflicts in the TCDT.

A computer program which embodies a process described herein is stored on at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided.

The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified according to the order of importance of the heuristic steps, in order to perform the described functions. All of these variations are considered a part of the claimed invention.

While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 

What is claimed is:
 1. A computer system for a hardware transactional memory (HTM), the system comprising: a processor core configured to: set a mode register of the processor core of a computer to indicate a HTM mode; execute a plurality of transactions by the processor core in the HTM mode based on the mode register; determine whether a first transaction of the plurality of transactions exceeds a failure limit of the processor core in the HTM mode; and based on determining that the first transaction exceeds the failure limit of the processor core in the HTM mode, transition the processor to an assisted transaction mode by setting the mode register of the processor core to indicate the assisted transaction mode.
 2. The system of claim 1, wherein determining that the first transaction exceeds the failure limit of the processor core in the HTM mode comprises: initializing a failure counter; and based on detecting an error during execution the first transaction: aborting and discarding the results of the first transaction; incrementing the failure counter based on the aborting of the first transaction; comparing the failure counter to a failure count limit of the processor core; and based on determining that the failure counter exceeds the failure count limit, determining that the first transaction exceeds the failure limit.
 3. The system of claim 2, further comprising, based on determining that the failure counter does not exceed the failure count limit: determining that the first transaction does not exceed the failure limit of the processor core in the HTM mode; and retrying execution of the first transaction in the HTM mode.
 4. The system of claim 2, wherein the error comprises one of a cache overflow and an unsupported operation.
 5. The system of claim 1, wherein transitioning the processor to the assisted transaction mode further comprises: aborting the plurality of transactions; starting an assist thread; retrying execution of the first transaction in the assisted transaction mode based on the mode register; and starting execution of any further transactions in the processor core in the assisted transaction mode based on the mode register.
 6. The system of claim 1, wherein execution of a current transaction in the assisted transaction mode comprises: determining an address for the current transaction in an address history table; determining whether an entry exists in a transaction conflict detection table for the address; based on determining that no entry exists in the transaction conflict detection table for the address, adding a new entry for the address to the transaction conflict detection table.
 7. The system of claim 6, further comprising: based on determining that the entry exists in the transaction conflict detection table for the address: determining whether the current transaction is marked as favored; based on determining that the current transaction is not marked as favored: determining whether the address is from a write; based on the address being from a write, determining whether there is another entry in the transaction conflict detection table for the address for another transaction; based on determining that there is another entry in the transaction conflict detection table for the address for another transaction, aborting the current transaction; based on determining that there is not another entry in the transaction conflict detection table for the address for another transaction, adding a process identifier of the current transaction to the entry in the transaction conflict detection table.
 8. The system of claim 7, further comprising, based on the address not being from a write, adding the process identifier of the current transaction to the entry in the transaction conflict detection table.
 9. The system of claim 7, further comprising, based on the current transaction being marked as favored: marking the entry in the transaction conflict detection table as favored; adding the process identifier of the current transaction to the entry in the transaction conflict detection table; and aborting any other transactions that are associated with the entry in the transaction conflict detection table.
 10. A computer program product for a hardware transactional memory (HTM), the computer program product comprising a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: setting a mode register of a processor core of a computer to indicate a HTM mode; executing a plurality of transactions by the processor core in the HTM mode based on the mode register; determining whether a first transaction of the plurality of transactions exceeds a failure limit of the processor core in the HTM mode; based on determining that the first transaction exceeds the failure limit of the processor core in the HTM mode, transitioning the processor to an assisted transaction mode by setting the mode register of the processor core to indicate the assisted transaction mode.
 11. The computer program product of claim 10, wherein determining that the first transaction exceeds the failure limit of the processor core in the HTM mode comprises: initializing a failure counter; and based on detecting an error during execution the first transaction: aborting and discarding the results of the first transaction; incrementing the failure counter based on the aborting of the first transaction; comparing the failure counter to a failure count limit of the processor core; and based on determining that the failure counter exceeds the failure count limit, determining that the first transaction exceeds the failure limit.
 12. The computer program product of claim 11, further comprising, based on determining that the failure counter does not exceed the failure count limit: determining that the first transaction does not exceed the failure limit of the processor core in the HTM mode; and retrying execution of the first transaction in the HTM mode.
 13. The computer program product of claim 11, wherein the error comprises one of a cache overflow and an unsupported operation.
 14. The computer program product of claim 10, wherein transitioning the processor to the assisted transaction mode further comprises: aborting the plurality of transactions; starting an assist thread; retrying execution of the first transaction in the assisted transaction mode based on the mode register; and starting execution of any further transactions in the processor core in the assisted transaction mode based on the mode register.
 15. The computer program product of claim 10, wherein execution of a current transaction in the assisted transaction mode comprises: determining an address for the current transaction in an address history table; determining whether an entry exists in a transaction conflict detection table for the address; based on determining that no entry exists in the transaction conflict detection table for the address, adding a new entry for the address to the transaction conflict detection table.
 16. A computer implemented method for a hardware transactional memory (HTM), comprising: setting a mode register of a processor core of a computer to indicate a HTM mode; executing a plurality of transactions by the processor core in the HTM mode based on the mode register; determining whether a first transaction of the plurality of transactions exceeds a failure limit of the processor core in the HTM mode; and based on determining that the first transaction exceeds the failure limit of the processor core in the HTM mode, transitioning the processor to an assisted transaction mode by setting the mode register of the processor core to indicate the assisted transaction mode.
 17. The method of claim 16, wherein determining that the first transaction exceeds the failure limit of the processor core in the HTM mode comprises: initializing a failure counter; and based on detecting an error during execution the first transaction: aborting and discarding the results of the first transaction; incrementing the failure counter based on the aborting of the first transaction; comparing the failure counter to a failure count limit of the processor core; and based on determining that the failure counter exceeds the failure count limit, determining that the first transaction exceeds the failure limit.
 18. The method of claim 17, further comprising, based on determining that the failure counter does not exceed the failure count limit: determining that the first transaction does not exceed the failure limit of the processor core in the HTM mode; and retrying execution of the first transaction in the HTM mode.
 19. The method of claim 17, wherein the error comprises one of a cache overflow and an unsupported operation.
 20. The method of claim 16, wherein transitioning the processor to the assisted transaction mode further comprises: aborting the plurality of transactions; starting an assist thread; retrying execution of the first transaction in the assisted transaction mode based on the mode register; and starting execution of any further transactions in the processor core in the assisted transaction mode based on the mode register. 